The prior art shows water filters wherein the water is caused to flow through a plurality of filtering layers, one of which layers is a porous membrane of submicron pore size. The submicron pore size membrane is an important element in such filters in that it not only blocks the passage of submicron deleterious solids such as asbestos, grit and the like, but it also blocks the passage of bacteria. The prior art also shows that such filters can be provided with valving whereby the water can be caused to bypass the filter whenever potable water is not required. This extends the life of the filter for providing potable water when it is needed for drinking and cooking. U.S. Pat. No. 4,025,438 shows such a filter.
But even with the bypass feature and the other features described in the aforesaid patent for extending the life of the filter, it does, nevertheless, have only a limited period of usefulness. And the limitation is due to the membrane, the useful life of which is tolled by one or the other, or both, of two conditions occurring.
One of the conditions which can arise is that the surface of the membrane becomes clogged with submicron or larger particles of any sort. Of course, where the water being filtered contains considerable solid material of sufficiently small particle size to escape filtration by earlier filter layers, this condition can arise after a relatively short period of filter use.
The other condition which can arise is by way of bacteria filtered from the water, and the growth or multiplication of the bacteria. It is a strange fact that even though a submicron pore size filter can block the flow of bacteria therethrough, nevertheless the bacteria can multiply and grow through the pores of the filter. That is, after some bacteria are captured on the upper or water entrance surface of the submicron pore size filter layer, they can, given the time to do so, culture and multiply progressively through the pores until they appear on the water exit surface of the filter layer, then to appear in water passed through the filter. The occurrence of this condition can be prevented or inhibited by the use of a bactericide in the filter; however, this has disadvantage in that no matter the conventionally used bactericides are substantially insoluble in cold water, they do have some solubility in warm water thereby giving rise to the possibility of being ingested if warm drinking or cooking water is drawn from the filter.